1. Field of the Invention
The present invention refers to a coffee brewing machine for preparing, alternatively, espresso coffee by a method in which hot water is forced under pressure through a compressed cake of coffee powder, or normal coffee by a method in which hot water runs without pressure through a filter containing coffee powder.
2. Prior Art
In known coffee brewing machines of this kind, practically two separate, different coffee brewing machines are united in one common housing: Firstly a coffee brewing machine for preparing normal coffee which obtains the fresh cold water from a common or a separate water tank or container. The fresh cold water is heated by a boiler or a geyser up to a temperature near the boiling point such that the hot water may flow through a pipe in a filter holder which has been provided with coffee powder. By flowing through the filter holder, the hot water is brewed up to coffee. Secondly a coffee brewing machine for preparing espresso coffee by a method by which hot water is forced under pressure through a compressed cake of coffee powder. In this machine, the fresh cold water is sucked from the same or a separate water tank or container by a pump and fed through a boiler or a geyser where it is heated up to the desired temperature to a brewing chamber filled with compressed coffee powder. The pressurized hot water penetrates the coffee powder cake and leaves the brewing chamber as espresso coffee.
Disadvantages of these known machines are that the constructional expenditure is very high since, for instance, two separate elements for heating up the water and two separate, differently located coupling elements, i.e. for the filter holder and for the brewing chamber which both have to be removably fixed, must be provided, and that the space requirements of such machines are very high.
Based on the knowledge that for preparing normal coffee on the one hand and for preparing espresso coffee on the other hand the feed rate of the hot water per time unit as well as the temperature thereof have to be adapted, it has been proposed in the German Auslegeschrift Nr. 1,778,604 to render variable the feed rate of &he pump which feeds the hot water or the heating power of the boiler or geyser. Even if thereby the possibility is given to adapt the parameters of operation of the coffee brewing machine manually, there is a great danger that the coffee brewing machine is incorrectly operated.